Combine the flour, cornmeal, onion powder, salt, milk, egg and 125ml/4fl oz water in a large bowl. Stir well until there are no lumps and the batter is smooth.
Slice the onions across to make rings about 1cm/1⁄2in thick.
Heat the oil in a large saucepan until hot. Test to see whether the oil is hot enough by dropping a little of the batter into the oil – it should sizzle straight away.
Dip the separated rings into the batter. Drop the coated rings into the oil and fry until golden brown. Cook in batches if necessary.
Remove the onion rings with a slotted spoon, and drain on kitchen paper. Serve hot.